This invention relates to light emitting devices and, more particularly, to a contact structure to the light emitting surface of such devices which typically are made from III-V compound semiconductors.
Light emitting devices as well as light collecting devices previously have used small distributed ohmic contacts in various arrays to enable electrical connection to the semiconductor material and, at the same time, to enhance light transmission at the contacted surface by reducing the area covered by the contact. With such small distributed contacts, means is provided to enable facile external connection to one or more of the contacts. One advantageous arrangement comprises a large area bonding pad which overlies several of the distributed contacts and to which a wire lead may be attached, typically by thermocompression bonding. However, it is desirable then to remove the distributed contacts not contacted by the bonding pad so as not to reduce light emission unnecessarily from the contacted surface.
A process to achieve the described contact structure on compound semiconductors such as gallium phosphide, gallium arsenide, and gallium aluminum phosphide carries a number of constraints. The distributed ohmic contacts, advantageously of gold with a small percentage of beryllium, are alloyed to the semiconductor material for good ohmic contact. They therefore are of both limited thickness and area. The larger area bonding pad usually is of substantially pure gold and is applied subsequent to the alloying step. To provide just a sufficient number of distributed contacts to be placed within the area of the bonding pad presents a mask registration problem. The use of chemical etching to remove both the distributed contacts outside the bonding pad or to define the relatively thick bonding pad, is time consuming and additionally is the source of contamination. Thus, an object of the invention is a process for making contact to the top or light emitting portion of a light emitting device with a high degree of reliability and with minimum interference with light emission.